1. Field of the Invention
This invention relates to a rotation control device for controlling the rotation of a rotary body which is arranged to cause a recording medium and a head which is provided for recording or reproduction on or from the recording medium to move relative to each other.
2. Description of the Related Art
The rotary head drum driving system of a helical scanning type video tape recorder or the like, for example, is required to allow a magnetic head mounted on a rotary head drum to scan recording tracks on a magnetic tape at a high speed and with a high degree of accuracy. To meet this requirement, the rotary head drum driving system of this kind has been provided with a servo-control device for controlling the rotation of the rotary head drum and that of a capstan or the like which causes the magnetic tape to travel in such a manner as to keep them under a given condition.
The drum servo-control for the rotary head drum has been performed in varied manners including, for example: A speed servo-control method in which a so-called FG pulse which gives a pulse signal according to the rotational frequency of the rotary head drum is detected and the speed servo-control is applied to the head drum to keep its rotational frequency at a given value according to the result of detection; and a phase servo-control method in which a so-called PG pulse which represents a rotation phase of the rotary head drum and the rotation of the rotary head drum is controlled to keep the rotation phase at a given value.
The speed servo-control method of using the FG pulse is suited for controlling the rotational frequency to the given value against such a great change that takes place at the start of the rotation of the rotary head or the like. The phase servo-control method of using the PG pulse is suited for controlling the rotation phase of the rotary head drum against its minor changes or fluctuations. With servo-control devices of these methods employed, the rotation of the rotary head drum can be highly accurately controlled.
Both the servo-control methods are carried out on the basis of position information (FG and PG) obtained by a device disposed at the rotary head drum or a head drum driving motor. Therefore, the rotation of the rotary head drum can be accurately stabilized for a recording operation according to these methods. However, in the case of reproduction, the adoption of these methods brings about jitters resulting in a transverse sway on a reproduced picture plane due to a positional deviation, inaccurate position information, a mechanical change in the recording medium (or tape) such as the elongation of the tape, and vibrations taking place in a tape transport system. The conventional rotation control which relies solely on the rotational-position information obtained from a rotary body thus has been incapable of effectively coping with time base changes that take place during reproduction.